Phoenix Crossland was sent off after his hip-drop tackle brought down Nathan Cleary in the 35th minute at Accor Stadium.
Cleary, a four-time premiership winner, was taken to hospital amid fears of a compound fracture in his left leg.
The packed crowd watched in shock as Cleary grimaced in pain on the ground.
"The stadium is silenced by this. He's had injuries in his career, but nothing like this, Nathan Cleary.
"I don't care who you support but that is a very sad moment for the game."
Cronk said, "He's in awful pain. It's a severe injury for Nathan Cleary,"
The injury also put Penrith's premiership hopes in doubt.